Optical disk technology is commonly used for storage of video, audio, and other digitally formatted information. A typical optical disk player system includes a combination of mechanical, optical, and electrical systems. The mechanical system is configured to clamp the disk in place and rotate the disk at a desired speed. The optical system is configured to retrieve the stored information from the rotating disk. The electrical system cooperates with the mechanical system to control disk loading and rotation. The electrical system also cooperates with the optical system to control detection and conversion of retrieved optical information to an electronic signal.
A compact disk (CD) is one type of optical disk that is used to store information. A typical CD consists of a transparent polycarbonate substrate that is covered by a reflective material, which in turn is covered by a protective layer. The substrate permits light to penetrate to the reflective layer. Information is stored in the reflective layer as a series of depressions or pits. The depressions in typical optical disks are between 110 and 130 nm in depth. The reflective layer is used to create a diffraction structure such that incident light of a given wavelength undergoes destructive interference. The interference decreases the intensity of reflected light at a predetermined distance that is a quarter wavelength of the incident light. The presence of pits in the surface of the reflective material are detected by photo-detectors in terms of the change in light intensity between the incident and reflected light. The length of the pits in a typical CD is approximately 830 nm.
A digital video disk (DVD is another type of optical disk that is used to store information. The DVD is similar in construction to a CD. However, a DVD can store information in up to four different reflective layers. Two of the reflective layers are on one side of the DVD, while the other two reflective layers are on the other side of the DVD. On each side of the disk, the second reflective layer is beneath the first reflective layer. As in CD technology, pits in the reflective layers are used to store information. The light source can be focused on either the first layer or the second layer. A photo-detector detects the presence of pits in the surface of the reflective layers by detecting the changes in light intensity between the incident and reflected light from the various reflective layers. The length of the pits in a typical DVD is approximately 400 nm, which is roughly half the length of pits in a typical CD. The typical storage capacity of a CD is around 700 MB, while a DVD can store about 4.38 GB for a single-sided/single-layer DVD, 7.95 GB for a single-sided/double-layer, 8.75 GB for a double-sided/single-layer DVD, and 15.9 GB for a double-sided/double-layer DVD.
A typical incident light source that is used in an optical disk system is a laser diode. The laser diode is located adjacent the surface of the disk, and focused at the surface of the reflective layer in the disk. In some instances, two different laser diodes are used in the same system. For example, some DVD players employ two laser diodes, where one laser diode is focused at one depth for reading DVDs, and another laser diode is focused at another depth for reading CDs.